In a typical ion implanter, a relatively small cross-section beam of dopant ions is scanned relative to a silicon wafer. This can be done in essentially one of three ways: scanning of the beam in two directions relative to a stationary wafer, scanning of the wafer in two directions relative to a stationary beam, or a hybrid technique wherein the beam is scanned in one direction whilst the wafer is mechanically scanned in a second, typically orthogonal direction.
Each technique has advantages and disadvantages. With smaller silicon wafers, the traditional approach was to mount a batch of wafers at the end of spokes on a rotating wheel. The wheel was then scanned to and fro to cause a fixed direction ion beam to impinge upon each wafer in turn.
For implantation into larger (300 mm) wafers, batch processing is currently not preferred. One reason for this is that the individual cost of each wafer introduces a significant financial risk should problems arise during implantation. Electrostatic or magnetic scanning of an ion beam in orthogonal directions relative to a stationary wafer tends to result in beams of poorer quality, and current single wafer scanning techniques tend to employ the hybrid mechanical/electrostatic scanning as outlined above. An arrangement suitable for achieving this is described in our commonly assigned U.S. Pat. No. 5,898,179, the contents of which are incorporated by reference in their entirety. Here, the ion beam is magnetically scanned in a first direction perpendicular to the beam line axis in the ion implanter, whilst the wafer is mechanically moved in a second, generally orthogonal axis.
There are, nevertheless, advantages (in terms of beam profile, beam stability and minimisation of the length of the beam line) in maintaining a static beam direction. This in turn requires dual direction scanning of the wafer. It is one object of the present invention to provide an arrangement which achieves this.
Determining the beam profile i.e., the ion density as a function of distance across the beam in a given direction) is generally desirable, but particularly when the beam is of fixed direction relative to the implantation chamber. This is because the speed of passage of the wafer across the beam is then slower than for hybrid scanning. For a reasonable throughput of wafers, therefore, it is necessary to minimise the raster pitch. It is then helpful to determine, for example, the beam profile (that is, the beam current intensity across the area of the beam) both prior to and during implantation. Profiling the beam prior to an implant allows the scanning of the wafer during implant to be controlled so as to ensure close uniformity across the wafer, rather than ‘stripes’ of lower or higher ion densities.
A number of different approaches to beam profiling are known in the art. For example, in our commonly assigned PCT Patent Application WO-A-00/05744, the signal output from the beam stop (located downstream of the batch processing wafer holder) is employed to obtain information on beam width, height and continuity during implantation. Such signal processing relies upon the gap between wafers on the rotary wafer holder and is accordingly not appropriate for single wafers.
Other beam profiling techniques include a travelling Faraday and a pair of Faradays held in a fixed position but spaced along the beam line as described in the above-referenced U.S. Pat. No. 5,898,179.
This invention also seeks to provide an improved ion beam profiling arrangement, therefore, particularly for use during set-up prior to implant.